Recently, various types of rear wheel steering apparatuses each constituting a portion of a four-wheel steering system (4WS) for a vehicle are known. Such rear wheel steering apparatus, for example, as disclosed in JP5098242B, which will be hereinafter referred to as Reference 1, basically includes a rod connected to a rear wheel of a vehicle, a housing supporting the rod and fixed to a chassis of the vehicle, and a motor accommodated within the housing and driving the rod to steer rear wheels. The rear wheel steering apparatus of Reference 1 is provided with a deceleration mechanism which converts a rotary motion of the motor to a linear motion of the rod, and transmits the motion. As an example of the deceleration mechanism, a planetary gear mechanism is disclosed in Reference 1. In addition, a planetary gear mechanism is applied to a rear wheel steering apparatus described in JP3068779B, which will be hereinafter referred to as Reference 2. According to Reference 2, for example, a ring gear constituting the planetary gear mechanism is made of resin material.
Further, according to a description of JP2012-511465A, which will be hereinafter referred to as Reference 3, a planetary gear mechanism is applied to a transmission for converting a rotary motion of an electric motor to a translational motion of a push rod, and a ring gear of the planetary gear mechanism is press-fitted to an outer ring of a thrust bearing of a ball screw. In addition, a steering device of Reference 3 is provided with a blocking device which connects/disconnects transmission of a drive force from the electric motor to the push rod. The blocking device is supported, together with the electric motor and the transmission, at a module housing.
At the apparatus and the device which are disclosed in Reference 1 and Reference 3, respectively, the ring gear of the planetary gear mechanism is made of metal and is press-fitted to the housing as a conventional mounting means. Thus, a process of press-fitting is needed, which is a factor for increasing costs. In a case where the ring gear is made of the resin material as described in Reference 2, the apparatus is made inexpensively, however, forming the ring gear inside the housing so that the ring gear is integral with the inside of the housing is not easy. The apparatus of Reference 2 is characterized in that gears are made of different resin materials from one another.
In addition, according to each of References 1 and 2, a structure for supporting the rod is a so-called both-ends-supported structure, that is, the rod is supported at both ends, and thus the supporting structure of the rod is complicated. In a case where the supporting structure of the rod is modified to be a one-end-supported structure where the rod is supported at one end, inefficiency occurs in an assembling process of other part or component of the device. On the other hand, the device described in Reference 3 may be categorized into the one-end-supported structure, however, a high strength is required because the device includes a structure where the electric motor, the transmission and the blocking device are supported at the module housing. This results in an increase in a size of the housing.
A need thus exists for a rear wheel steering apparatus for a vehicle, which is not susceptible to the drawback mentioned above.